Subacute Combined Degeneration (SCD)

Biochemistry and Mechanism

1. Methylmalonyl-CoA → Succinyl-CoA (requires adenosylcobalamin)
   • Deficiency leads to accumulation of methylmalonic acid (MMA)
   • MMA may disrupt myelin synthesis by substituting abnormal fatty acids
2. Homocysteine → Methionine (requires methylcobalamin + methionine synthase)
   • Methionine is converted to S-adenosylmethionine (SAM)
   • SAM is the universal methyl donor for methylation reactions in the nervous system
   • Without SAM, myelin maintenance fails
   • Homocysteine accumulates (neurotoxic, also cardiovascular risk)

• Bradley & Daroff's Neurology in Clinical Practice, p. 1796

Causes of Vitamin B12 Deficiency

• Bradley & Daroff's Neurology, p. 1798; Harrison's 22E, p. 1903

Pathology

1. Posterior (dorsal) columns at cervical and upper thoracic levels - most severely affected
2. Lateral columns (corticospinal tracts)
3. Anterior columns - only in advanced disease

• Bradley & Daroff's Neurology, p. 535

Clinical Features

Symptoms (roughly in order of onset):

• Paresthesias (tingling, numbness) in hands and feet - often the first symptom
• Loss of vibration sense and proprioception (posterior column)
• Lhermitte's sign may occur

• Progressive spastic weakness (lateral column / corticospinal)
• Sensory ataxia → positive Romberg sign
• Loss of reflexes (from peripheral neuropathy) + Babinski signs (from corticospinal involvement) - this coexistence is a classic diagnostic clue

• Optic atrophy
• Cognitive changes - irritability, memory impairment, dementia
• Cerebral white matter involvement

• Harrison's 22E, p. 1903; Bradley & Daroff's Neurology, p. 1274

Diagnosis

Blood Tests

• Goldman-Cecil Medicine, p. 3681; Bradley & Daroff's Neurology, p. 938

Neurophysiology

• Nerve conduction studies: small or absent sural nerve sensory potentials (axonal polyneuropathy) in ~50% of patients
• Somatosensory and visual evoked responses: nonspecific abnormalities
• EEG: nonspecific

MRI Spinal Cord

• T2 hyperintensity in the dorsal columns, sometimes also lateral columns
• Lesions most often at C2-C5 level, bilateral and symmetric
• Described as an "inverted V" or linear hyperintensity on axial T2
• T1: may show hypointensity in dorsal columns ± mild cord enlargement
• Gadolinium enhancement: variable (some series show enhancement, others do not)
• MRI may be normal - a negative MRI does not exclude SCD
• Grainger & Allison's Diagnostic Radiology, p. 215; Bradley & Daroff's Neurology, p. 530

Closely Related Conditions

1. Nitrous Oxide (N2O) Myelopathy

• N2O irreversibly oxidizes Co(I) to Co(III) in cobalamin, permanently inactivating methionine synthase
• Produces a myelopathy identical to SCD
• Can occur after a single prolonged anesthetic exposure, especially in patients with marginal B12 stores
• Also from recreational abuse ("whippets")
• Prevention: B12 injections weeks before elective N2O anesthesia in at-risk patients

2. Copper Deficiency Myelopathy

• Clinically and radiologically indistinguishable from SCD but serum B12 is normal
• Low serum copper and ceruloplasmin
• Causes: post-bariatric surgery, excess zinc (supplements, denture creams) - zinc induces metallothionein which sequesters copper
• Treatment: oral copper supplementation 2 mg/day

3. Folate Deficiency Myelopathy

• Similar but milder than SCD; rarely seen since dietary fortification programs
• Always exclude B12 deficiency before treating with folate alone
• Harrison's 22E, p. 1904-1905; Bradley & Daroff's Neurology, p. 535

Treatment

1. 1000 µg IM daily x 5-7 days
2. Then 1000 µg IM weekly x 4-8 weeks (1 month)
3. Then 1000 µg IM monthly (lifelong maintenance for pernicious anemia)

• High-dose cyanocobalamin 1-2 mg orally daily - even in pernicious anemia, passive diffusion (1-5%) absorbs enough at this dose
• Suitable for maintenance once neurological stabilization achieved; requires monitoring

• Serum MMA and homocysteine normalize within ~2 weeks
• Neurological improvement is delayed and may be incomplete
• Most symptomatic improvement occurs in the first 6-12 months
• Prognosis correlates inversely with duration of symptoms before treatment - early diagnosis is key
• If symptoms progress despite normalized B12, consider copper deficiency
• Harrison's 22E, p. 1903; Bradley & Daroff's Neurology, p. 547

Differential Diagnosis

Summary

• A 2024 review (Holroyd & Berkowitz, Continuum) covers metabolic myelopathies including SCD with updated diagnostic approach.
• A 2025 comprehensive review (Bernetti et al., Brain Sci) specifically addresses SCD MRI diagnostic challenges and treatment pathways.
• Recreational nitrous oxide misuse as a cause of SCD is an increasing clinical concern, highlighted in a 2025 review (PMID 40901226).

Radial Nerve Injury - Clinical Presentation & Physiotherapy Management

Anatomy Recap (Clinically Relevant)

• Superficial branch (purely sensory) - dorsum of hand and digits
• Deep branch / Posterior Interosseous Nerve (PIN) - enters the supinator canal, supplies all extensor muscles of the forearm

Causes by Level

• Thieme Atlas of Anatomy, p. 388-389; Rockwood & Green's Fractures 10th Ed 2025, p. 789

Clinical Presentation by Level of Injury

1. High Radial Nerve Injury (Axilla)

• Triceps (elbow extension) - weak/paralysed
• Brachioradialis
• All wrist extensors (ECRL, ECRB, ECU)
• Finger extensors (EDC, EI, EDM)
• Thumb extensors and abductor (EPL, EPB, APL)

• Posterior cutaneous nerve of arm (dorsum of arm)
• Posterior cutaneous nerve of forearm
• Superficial radial nerve territory: dorsum of hand (radial 3½ digits), first dorsal web space (most consistent/exclusive area)

2. Spiral Groove (Most Common) - Mid-Humeral Injury

• Triceps spared (branches to triceps leave before the groove)
• Brachioradialis weak/paralysed
• All wrist and finger extensors paralysed
• Thumb extensors/abductor paralysed

• Dorsum of hand and digits (superficial branch territory)
• Posterior cutaneous nerve of arm spared (branches before groove)

Diagnostic tip: Test brachioradialis (forearm in mid-prone position, flex elbow against resistance - muscle stands out). If brachioradialis is intact, injury is below the spiral groove.

3. Posterior Interosseous Nerve (PIN) / Distal Injury

• Wrist extension partially preserved (ECRL and ECRB supplied before PIN enters supinator)
• Finger extension at MCP joints lost (EDC, EI, EDM)
• Thumb extension and abduction lost (EPL, EPB, APL)
• No wrist drop (wrist can extend, but deviates radially due to preserved ECRL/ECRB)

4. Superficial Radial Nerve (Wartenberg's Syndrome)

Hallmark Sign: Wrist Drop

• Inability to actively extend the wrist (hangs in flexion)
• Inability to extend MCPJs of fingers (can extend IPJs via interossei - ulnar nerve)
• Inability to extend/abduct thumb (EPL, EPB, APL)
• Key clinical pearl: IP joint extension via the interossei (ulnar nerve) remains intact - do NOT mistake this for nerve recovery
• S. Das Manual on Clinical Surgery 13th Ed; Thieme Atlas, p. 387

Investigations

• If no clinical recovery at 3 weeks: perform EMG/NCS
• Repeat at 3 months if no recovery
• At 6 months without recovery: consider surgical exploration
• Harrison's 22E, p. 999; Campbell's Operative Orthopaedics 15th Ed 2026

Natural History and Prognosis

• Spontaneous recovery: 70% or more of cases
• Mean time to onset of recovery: 7 weeks (range 2 weeks to 6 months)
• Mean time to full recovery: 6 months (range 3-12 months)
• Transection (neurotmesis) is rare - mainly with open fractures
• Rockwood & Green's Fractures, p. 789-790

Physiotherapy Management

Phase 1: Acute / Early Phase (0-6 weeks)

1. Splinting (Orthotic Management)

• Cock-up wrist splint (wrist extension splint, 20-30° extension): the primary splint. Prevents wrist flexion contracture, maintains functional position for grip
• Dynamic extension splint (lively splint): allows passive wrist/finger flexion with active elastic-assisted extension - preferred for daytime functional use as it enables some hand function
• Splint worn most of the day; removed for exercise sessions

Harrison's 22E: "Treatment consists of cock-up wrist and finger splints, avoiding further compression, and physical therapy to avoid flexion contracture."

2. Passive Range of Motion (PROM) Exercises

• Daily passive mobilisation of all joints in the affected limb:
  • Wrist: flexion/extension/deviation
  • MCP and IP joints of fingers
  • Thumb CMC, MCP, IP joints
  • Elbow and shoulder
• Goal: prevent flexion contractures and maintain full joint range
• 2-3 times daily, 10-20 repetitions each

3. Oedema Management

• Elevation of the limb
• Retrograde massage
• Compression bandaging if oedema present

4. Sensory Protection and Education

• Educate patient to avoid heat, pressure, trauma to anaesthetic skin
• Avoid tight watch straps, jewellery, sleeping on the arm
• Regular skin inspection

5. Maintain Proximal Strength

• Active exercises for unaffected muscles (shoulder, elbow flexion/pronation-supination)
• Prevent disuse atrophy of proximal muscles

Phase 2: Recovery Phase (6 weeks - 6 months)

1. Monitor Recovery

• Track Tinel's sign progression distally along the nerve course (sign of axonal regeneration)
• Serial manual muscle testing (MRC grading)
• Re-check EMG at 3 months if no improvement

2. Active Assisted and Active Exercises

• As soon as any active wrist/finger extension returns (MRC grade ≥2):
  • Active-assisted wrist extension
  • Active extension of MCPJs
  • Thumb extension and abduction
• Progress from gravity-eliminated positions → gravity-resisted positions
• Use mirror therapy or biofeedback to facilitate motor re-education

3. Muscle Re-education Techniques

• Facilitation techniques: tapping, vibration, quick stretch over wrist extensors
• Proprioceptive Neuromuscular Facilitation (PNF): patterns involving radial extensors (e.g., D2 extension pattern)
• Electrical stimulation / Neuromuscular electrical stimulation (NMES/FES): applied to wrist and finger extensors to maintain muscle bulk, prevent atrophy, and facilitate re-education of recovering muscles
• EMG biofeedback: visual/auditory feedback of muscle activity to reinforce voluntary effort

4. Strengthening Exercises

• Progressive resistive exercises for extensors as grades improve
• Theraband / putty exercises for wrist extension
• Grip strengthening (flexors work against gravity with wrist supported)

5. Sensory Re-education

• Once sensation starts recovering:
  • Desensitisation (textures, vibration, temperature discrimination)
  • Sensory discrimination training (moving vs. static two-point discrimination)
  • Progression from coarse to fine textured materials

Phase 3: Functional Rehabilitation (3-12 months)

1. Functional Task Training

• Activities of daily living (ADL) training: writing, dressing, food preparation
• Occupational tasks relevant to the patient's work
• Progressive difficulty of task demands

2. Strengthening and Endurance

• Resistive wrist extension with free weights, Theraband
• Finger extension exercises against resistance
• Grip and pinch strengthening (dynamometer-guided)

3. Coordination and Dexterity

• Pegboard exercises, manipulation of small objects
• Fine motor tasks: buttoning, picking up coins

4. Work/Sport-Specific Rehabilitation

• Simulate occupational demands
• Return-to-sport conditioning as appropriate

Electrotherapy Modalities

Indications for Surgical Referral (from Physiotherapy Perspective)

• No clinical improvement (Tinel's sign not advancing) after 6-8 weeks of conservative management
• EMG at 3 months shows no reinnervation
• Open fracture / penetrating injury (early exploration indicated)
• Evidence of nerve entrapment or rupture on ultrasound/MRI
• No functional recovery at 6 months

• Rockwood & Green's Fractures 10th Ed 2025; Harrison's 22E, p. 999; Campbell's Operative Orthopaedics 15th Ed 2026

Summary Table: Physiotherapy by Phase

Differential Diagnoses for Wrist Drop